Bob,
Clear, you have a lot more experience and knowledge. For me this is
typical a case of "If you don't know about something it must be simple"
So best would be to make it possible in the simple PNA to switch off the
opamp gain, without changing the impedance the mixer sees, so the
offset tuned signal can be used to calibrate the slope.
I found this picture very helpful to understand the relation between
phase modulation depth and the strength of the side bands
http://athome.kaashoek.com/time-nuts/PM_Sidebands.JPG
It shows that below 0.2 radian peak phase modulation you can simplify to
narrowband FM as only the 1st sideband has relevant power (certainly for
the accuracy I am after)
The whole presentation including the calculation can be found here:
http://athome.kaashoek.com/time-nuts/Measuring_phase_modulation.pdf
Written by Bob Nelson from Keysight.
Very helpful presentation for people (like me) that are new to all this.
Erik.
On 8-7-2022 18:58, Bob kb8tq wrote:
Hi
Like it or not, the mixer is a non-linear load. It also has a frequency
dependence. Even with “saturation” levels, the slope can and does
change. That’s the short list, as you dive into it, things get even more
complex in terms of “might be” sort of issues.
How can you be in saturation and have the slope change ( it does sound
unreasonable) ? The fundamental is not changing much (so you are
in saturation). The harmonics of the fundamental are changing. Since
the output is actually a triangle wave with rounded “corners” there are
indeed harmonics very much present.
The flat parts of the triangle wave are a “good thing” in this case. It
makes the device linear over a bit wider range than a sine wave would
provide. This gets you out of all sorts of nutty analysis concerning the
noise being “to much” to measure with the device. It also relaxes the
needed accuracy of the DC lock part of things. ( = slope of a sine wave
changes quickly ….).
You never really get away from the “to much noise” question. The
common definition of phase noise is that it’s more than 60 db below
carrier. That is really just the commonly used limit for “you may need
to think about FM sidebands”. Yes, that’s another rabbit hole to wander
down ….
Bob
On Jul 8, 2022, at 8:32 AM, Erik Kaashoek <[email protected]> wrote:
Bob
This confuses me.
The calibration of the system changes ( or can change ) each and every time you
swap
out signal sources. The levels are not going to be consistent setup to setup.
Thus you
calibrate each and every time you change out either device.
Assuming each source is saturating the mixer sufficiently (to be confirmed by
measuring the output level of the source into 50 ohm) I do not understand how
changing a source can change the calibration. Can you explain what is happening?
Please keep in mind I'm not after 0.1dBc/Hz accuracy, +/- 5dBc/Hz would already
be great.
Erik.
_______________________________________________
time-nuts mailing list -- [email protected]
To unsubscribe send an email to [email protected]
